FIG. 1 is a longitudinal section view showing an induction motor in accordance with the conventional art, and FIG. 2 is a cross-section view showing the induction motor of FIG. 1.
Referring to FIGS. 1 and 2, the conventional induction motor comprises a motor casing 10 that forms an appearance; a stator 100 disposed on an inner circumferential surface of the motor casing 10, and having a coil wound thereon in a circumferential direction; an induction rotor 20 rotatably inserted into the stator 100; and a magnetic rotor 30 rotatably inserted between the stator 100 and the induction rotor 20.
The motor casing is implemented as a cylindrical-shaped container having an opening. A cover 10a that covers the opening is coupled to the motor casing 10. A mounting groove 10b is disposed at a lower side of the motor casing 10. A bearing 12 that rotatably supports a rotation shaft 11 is disposed at the mounting groove 10b. 
The stator 100 includes a stator core 110 having a predetermined length, and a winding coil 120 wound in the stator core 110 in a circumferential direction.
The stator core 110 is a laminator formed as a plurality of sheets are laminated to one another. The stator core 110 includes a yoke portion 111 having a ring shape of a predetermined width, and a plurality of teeth 112 extending from an inner circumferential surface of the yoke portion 111 with a predetermined length. A slot is formed between the teeth 112 and the teeth 112, and a cavity 110a for inserting the induction rotor 20 into the stator core 110 is formed by each end of the teeth 112.
The winding coil 120 is wound around the teeth 112 with plural times, and is disposed on the slot 113 formed between the teeth 112 and the teeth 112.
The induction rotor 20 includes a rotor core 21 having a cylindrical bar shape of a predetermined length, and a conductor bar 22 inserted into the rotor core 21. The induction rotor 20 is inserted into the cavity 110a of the stator 100.
The rotor core 21 is a laminator formed as a plurality of sheets are laminated to one another. The rotation shaft 11 is coupled to the center of the rotor core 21.
The magnetic rotor 30 includes a magnet 31 having a cylindrical shape of a predetermined thickness, and a holder having a cup shape and supporting the magnet 31. The magnet 31 is rotatably inserted between an inner circumferential surface of the cavity 110a of the stator 100 and an outer circumferential surface of the induction rotor 20.
A bearing groove 32a is disposed at a lower side of the holder 32, and a bearing 33 which rotatably supports the rotation shaft 11 is coupled to the bearing groove 32a. Once the bearing 33 is coupled to the rotation shaft 11, the holder 32 can be freely rotated centering around the rotation shaft 11.
Hereinafter, an operation of the conventional induction motor will be explained.
When power is supplied to the winding coil 120 of the stator 100, a rotating magnetic field is formed. By the generated rotating magnetic field, the magnet rotor 30 is rotated at a synchronous speed.
As the magnetic rotor 30 constituted with a magnet is rotated, a rotating magnetic field having an intensive magnetic flux is generated. The induction rotor 20 is rotated by the generated rotating magnetic field.
As the induction rotor 20 is rotated, the rotation force of the induction rotor 20 is transmitted to parts requiring the rotation force through the rotation shaft 11.
However, the conventional induction motor has the following problems. The conventional induction motor is not sensitive to variation of an external power due to the intensive magnetic flux generated by the magnet 31 of the magnetic rotor 30, so that it is always operated at the same speed. The conventional method such as a voltage phase control requiring a low cost can not be used to vary the rpm of the induction motor. That is, since the induction motor has a limitation in a speed variation, it can not be operated at various speeds without an expensive inverter driving apparatus.